Download Applications of Low Power Current and Voltage Sensors

CIRED
21st International Conference on Electricity Distribution
Frankfurt, 6-9 June 2011
Paper 0822
APPLICATIONS OF LOW POWER CURRENT AND VOLTAGE SENSORS
Rolf FLURI, Joachim SCHMID, Paul BRAUN
TRENCH Switzerland AG – Switzerland
rolf.fluri@trench-group.com
joachim.schmid@trench-group.com
paul.braun@trench-group.com
ABSTRACT
Protection relays, meters and control units in up-to-date
switchgear are built with digital technology. The modern
secondary equipment does not need the high power output
of the instrument transformers (ITs) as this was necessary
for electromechanical relays. As a consequence the requirements for ITs changed. Requests for a universal use of
ITs for measurement, protection and even metering applications and the reduction of the overall costs of switchgear
are common. Further requirements to modern ITs beside
reliability and safety are easy handling, the reduction of the
expenditure of work during planning and installation and
short lead times.
By using established and highly proven components such as
iron cores, capacitors, resistors and quadripol shunts it is
possible to produce sensors which are fulfilling all the requirements. Compared with traditional ITs, low power ITs
have less overall costs, lower weight and are immune to the
electromagnetic interference in the substation. One low
power IT can be used for metering and protection purposes.
For that reason the number of different types for all application can be dramatically reduced in a wide range of primary current.
Nowadays relays with inputs for low power ITs are available from several relay manufacturers. By using such relays
together with the wide measuring range of low power ITs
users get a system with a high reliability, functionality and
flexibility.
The paper has its focus on the applications of low power
current and voltage sensors, such as the optimization of
switchgear in dimensions, weight and costs, power measurement in a compressor test field and new possible applications (i.e. DC measurement, Power Quality Measurement).
INTRODUCTION
This paper describes low power current and voltage sensors
which are in accordance to the IEC standards 60044-7 or
60044-8. It explains the different principles of low power
ITs and describes features and advantages of the technology. The main part describes miscellaneous applications
which have been developed, tested and installed during the
last several years.
The requirements regarding power, primary current, measuring- and protection- accuracy classes make it necessary
that conventional ITs need to be dimensioned always newly
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for each application. On the other hand, low power ITs with
their wide measuring range can be used for a big number of
different applications without the need of new dimensioning. They are compatible with the modern microprocessor
controlled relays and other secondary equipment and can be
used at medium- and high-voltage applications to replace
conventional ITs for measurement and protection purposes.
Low power ITs fit into existing as well as future designs of
switchgear.
LOW POWER SENSORS
Microprocessor-based relays are self-powered and have
high input impedance. For this reason their burdens to ITs
are very low. Modern low power sensors for voltage measurement according IEC 60044-7 [1] and for current measurement according IEC 60044-8 [2] have been developed
during the last decades. They are not only providing the
same functionality as conventional ITs but have a lot of
additional advantages. They are smaller concerning size and
weight and for this reason handling is easier and less space
is required in the switchgear. Low power sensors have a
very good linearity and an extended measuring range. Both,
current and voltage sensors provide a low voltage signal
output (mV to some Volts). Thereby the safety for the connected equipment and for the field service staff increases
and the risk of damages caused by human errors is reduced.
Low Power Current Transformers
The Low Power Current Transformer (LPCT) operates on
the proven instrument transformer principle basing on the
specific matching to an internal shunt resistor [3]. This
principle is exceptionally non-sensitive to external stray
fields. The secondary current produces a voltage across the
internal shunt resistor which is directly proportional to the
primary current. This voltage is the output signal of the
LPCT. The shunt itself is designed to withstand short circuit
currents without any change of its resistance value. LPCTs
operate linearly and saturation-free up to the short circuit
currents.
Compensated Low Power Voltage Transformers
The compensated Low Power Voltage Transformer (LPVT)
consists of a resistive voltage divider. The separate resistors
have a very low inductance. The resistors of the primary
side are “zig-zag” or spirally arranged [4]. For voltage levels between 1 and 72 kV the resistors are embedded into a
casted resin. The “zig-zag” arrangement and the permittivity
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CIRED
21st International Conference on Electricity Distribution
Frankfurt, 6-9 June 2011
Paper 0822
of the resin result in a capacitance, which is bigger than the
stray capacitance to ground. For this reason the influence of
the stray capacitance to the accuracy is compensated and the
behaviour within a defined frequency range is improved
(see also chapter about Power Quality Measurement).
LPVTs do not contain ferromagnetic material and for this
reason they cannot have ferroresonance. They are linear
over a large voltage range. The linearity allows reducing the
number of different required types and as a result the order
specific engineering can be reduced.
one phase low power ITs with dimensions according to DIN
42600, part 8 (see figure 2) have been installed.
Also a combination of a current and a voltage transformer is
possible and described in the following applications.
Figure 2-Combined instrument transformer 12 kV- Type
LPVCT (LPVT and LPCT) - used for power measurement
APPLICATIONS
For the evaluation and calculation, the sensors are connected to a digital power meter with a high-speed data acquisition rate. Figure 3 shows the general set up for detecting, recording and processing the data.
Since launching the low-power instrument transformer
technology in 1999 [4],[5] numerous development projects
were conducted to allow the installation of the technology
for different new applications.
Power Measurement
To evaluate the efficiency of a compressor either the mechanical or the electrical power measurement method can be
used. In the case described in the following, this measurement was done first mechanically by torque and speed
measurement. Due to the lack of some measuring ranges
during the test sequence the user decided to upgrade the test
stand with an additional electrical power measurement system in parallel to the mechanical power measurement system. With the additional electrical measurement it is not
only possible to measure the whole range but also to compare the values from the different measurement methods.
Thus, also mismeasurements can be detected and avoided.
Fig. 1 shows a block diagram of the upgraded compressor.
Figure 1-Block diagram of a compressor test stand with
electrical and mechanical power measurement
Mainly because of the following reasons the user decided to
install low power sensor technology for the electrical power
measurement:
- Lower costs compared to comparable measurement systems.
- Limited space inside the switchgear.
- Easy exchange of the existing conventional current
transformers by the new low power sensors.
- Accurate measurement in the frequency range between 25 Hz and 300 Hz.
- Harmonic measurement up to 2500 Hz possible.
For the described three phase application three combined
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Figure 3-General set up of the electrical power measurement, bus system and data acquisition
A comparison of the mechanical and electrical power measurement confirmed that the electrical power measurement
works accurately and reliably.
The required overall accuracy of the electrical power measurement of 2% is fulfilled at 50 Hz down to a power factor
cos of 0.3 and at 300 Hz down to power factor cos of
0.8. The electrical power measurement system (LPVCT and
wattmeter) was also able to measure harmonics up to 2500
Hz (i.e. caused by the use of a frequency converter).
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CIRED
21st International Conference on Electricity Distribution
Frankfurt, 6-9 June 2011
Paper 0822
DC Measurement
Another application for LoPo voltage sensors is the measurement of a DC voltage. LOPO sensors have been installed
to monitor the voltage across a capacitor bank discharged
by resistors after disconnected from the bus. The mechanical switched capacitor bank is normally connected to an
AC-bus that is running at up to 10 kV AC RMS. Depending
on the kind of monitoring device connected to the LPVT, it
is possible to detect both, the AC voltage during normal
operation and the decreasing DC voltage during the discharge of the capacitor bank. The accuracy was for AC
lower than 3% and for DC below 0.1%.
Figure 4 shows the equivalent circuit of such an installation.
Figure 6-Typical frequency response behaviour (ratio error)
of a LPVT
Better frequency response behaviour can be achieved with
resistive-capacitive voltage dividers (RC-dividers). By
adding capacitors in parallel to the primary and secondary
resistor columns of the R-divider, the influence of the stray
capacitance is significantly reduced. The transfer function
for an RC-divider [8], see formula (1), shows that the ratio
of the divider is independent of the frequency if the formula
(2) is valid.
US
RS


U P R  R 1  RS jCS 
S
P
1  RP jCP 
Figure 4-Equivalent circuit of voltage measurement with
LPVT at a capacitor bank
Power Quality Measurement
One of the requirements to low power ITs is that the behaviour over a certain frequency range is comparable with the
behaviour of the conventional inductive CTs and VTs. For
the measurement of power quality parameters according to
IEC 60000-4-30 in medium- and high voltage networks the
use of “measurement transducers” ([6], p 21) is necessary,
i.e. voltage sensors or voltage transformers.
Measurements on LPVTs and LPCTs confirmed that they
are basically suitable to detect the parameters up to the
required 50th harmonic [7].
Figures 5 and 6 illustrate measurements for the verification
of the suitability of low power CTs and VTs for the power
quality measurement. As can be seen, good frequency behaviour can be expected up to several tens of Kilohertz for
LPCTs and several thousand of Hertz for the LPVTs.
CP


1
1 

j

R
C
S S 

CP  CS


1

1 
jRP C P 

(1)
(2)
RS  C S  R P  C P
The RC divider is then fully compensated and can be used
for measurements starting from DC up to several tens of
Kilohertz (for special applications up to several Megahertz).
Hence, it can be used for power quality measurement including the measurement of transient voltages.
Earth-Fault Measurement
A special wiring allows using the LPVT for earth fault
protection in three phase networks. For this application the
output signals of all three LPVTs are interconnected in
parallel as shown in figure 7.
Figure 7-Scheme for earth fault measurement with LPVTs
Figure 5-Typical frequency response behaviour of a LPCT
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Without any earth faults in the system, the output voltage Us
can be calculated with the following formula:
U 1 U 2 U 3  0
(3)
1
1
1
1
1




R2 g R2.1 R2.2 R2.3 Rin
(4)
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CIRED
21st International Conference on Electricity Distribution
Frankfurt, 6-9 June 2011
Paper 0822
with Rin: Input impedance of the meter
R1.1  R1.2  R1.3  R1 (assumption)
(5)




R1   I 1  I 2  I 3   3  U s  0







I s .tot


U s  R2 g  I s.tot
(7)
R1  I s.tot  3  R2 g  I s.tot  0
(8)
I s.tot  0  U s  0
(9)
(6)
Conclusion: Without any earth fault, the output voltage U s
is zero.
With an earth fault, the output voltage U s is:
U1 U 2 U 3  0  U F




R1   I 1  I 2  I 3   3  R2 g  I s.tot  U F







I s .tot


I s.tot  R1  3  R2 g  U F


U s  R2 g  I s.tot
Us 
R2 g
R1  3  R2 g
(10)
(11)
(12)
(13)
U F
(14)
Conclusion: In case of an earth fault, an output voltage
proportional to the vector sum of the residual phase voltages
is measured.
Switchgear Optimisation
Due to the reduced dimensions and weights of low power
sensors new application fields can be defined leading to
switchgear designs with reduced size and overall cost. Figure 8 shows a combined unit where the LPCT and LPVT
sensors are integrated into an epoxy resin cast bushing.
Figure 8-Combined voltage and current sensors in an epoxy-resin bushing (Ur=12 kV, Ir=2500 A)
Bushings with combined low power sensors withstand short
circuit currents up to 31.5 kA, 3 s and ambient temperatures
up to 80°C. The standard accuracy class is 0.5 and 0.2 as an
option for voltage and current measurement. The connection
cable with connector (“RJ45”) is part of the sensor. The
signals from both sensors (VT and CT) are transmitted by
only one cable to the relay.
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Other integrations of LPVTs into the substation are possible. Figure 9 shows the integration into a GIS bus barsystem as well as the implementation into a support insulator for air insulated switchgear.
LPVT-G
b)
a)
Figure 9-a) LPVT integrated into a 36 kV GIS-bus system
and b) implemented into a 24 kV support insulator
CONCLUSION
Compared to conventional ITs, low power current and voltage sensors show some crucial advantages. The main advantages are the wide linearity, the high frequency range,
the small size and the low weight. This further leads to less
customisation effort and reduces the huge number of different required types of instrument transformers.
The application of this new technology allows designing
switchgear with optimized size and cost. In addition further
applications are possible such as power measurement, DCmeasurement and Power Quality measurement.
REFERENCES
[1] IEC 60044-7, 1999, “Electronic voltage transformers”.
[2] IEC 60044-8, 2002, „Electronic current transformers“.
[3] R. Minkner, 2003, “Universeller Ringkern Stromwandler für Mess- und Schutzzwecke”, etz Elektrotech. +
Autom., vol. 22, 2-9.
[4] R. Minkner, J. Schmid, 1999, “Koaxiale Spannungssensoren in gasisolierten Mittelspannungsanlagen”, etz
Elektrotech. + Autom., vol. 19, 62-64.
[5] R. Minkner, E.O. Schweitzer, 1999, “Low Power Voltage and Current Transducers for Protecting and Measuring Medium and High Voltage Systems”, 26th Western Protective Relay Conference (WPRC), Spokane,
Washington/USA.
[6] IEC 61000-4-30, 2003, “Electromagnetic compatibility
(EMC)–Part 4-30: Testing and measurement techniques–Power quality measurement methods”
[7] IEC 61000-2-4, 2003, “Electromagnetic compatibility
(EMC)– Part 2-4: Environment – Compatibility levels
in industrial plants for low-frequency conducted disturbances”
[8] R.Minkner, 2002, “Ein universeller RC-Spannungswandler für Hochspannungs Versorgungsnetze”, etz
Elektrotech. + Autom., vol. 22, 2-9.
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